The delivery of radio frequency (RF) energy to target regions within solid tissue is known for a variety of purposes of particular interest to the present invention. In one particular application, RF energy may be delivered to diseased regions (e.g., tumors) for the purpose of ablating predictable volumes of tissue with minimal patient trauma.
RF ablation of tumors is currently performed using one of two core technologies. The first technology uses a single needle electrode, which when attached to a RF generator, emits RF energy from an exposed, uninsulated portion of the electrode. The second technology utilizes multiple needle electrodes, which have been designed for the treatment and necrosis of tumors in the liver and other solid tissues. U.S. Pat. No. 6,379,353 discloses such a probe, referred to as a LeVeen Needle Electrode™, which comprises a cannula and an electrode deployment member reciprocatably mounted within the delivery cannula to alternately deploy an electrode array from the cannula and retract the electrode array within the cannula. Using either of the two technologies, the energy that is conveyed from the electrode(s) translates into ion agitation, which is converted into heat and induces cellular death via coagulation necrosis. The ablation probes of both technologies are typically designed to be percutaneously introduced into a patient in order to ablate the target tissue.
In theory, RF ablation can be used to sculpt precisely the volume of necrosis to match the extent of the tumor. By varying the power output and the type of electrical waveform, it is possible to control the extent of heating, and thus, the resulting ablation. However, the size of tissue coagulation created from a single electrode, and to a lesser extent a multiple electrode array, has been limited by heat dispersion. Increasing generator output, however, has been generally unsuccessful for increasing lesion diameter, because an increased wattage is associated with a local increase of temperature to more than 100° C., which induces tissue vaporization and charring. This, then, decreases the moisture concentration of the tissue, and increases local tissue impedance, limiting RF deposition, and therefore heat diffusion and associated coagulation necrosis.
Currently, RF generators are designed to minimize the time required to create large ablation volumes, while avoiding tissue vaporization and charring. These RF generators output and increase/decrease energy (i.e., power, current, voltage) in a consistent and steady manner. For example, as illustrated in FIG. 1, an energy output from an exemplary RF generator is maintained at a constant high level until a rise in tissue impedance or temperature indicating that an endpoint (i.e., largest volume of desiccated tissue with minimal to no charring) has been reached, after which the energy output steadily decreases. As another example, as illustrated in FIG. 2, an energy output from an exemplary RF generator steadily increases until a rise in tissue impedance or temperature indicated that an endpoint has been reached, after which the energy output steadily decreases.
While these RF generators efficiently provide large ablation volumes without tissue charring, it would be desirable to further decrease the ablation procedure time.